Rosemount 8732
Reference Manual
x: Input flow signal from sensor.
o: Average flow signals and transmitter output, determined by the “number of samples” parameter.
Tolerance band, determined by the “percent limit” parameter.
–Upper value = average flow + [(percent limit/100) average flow]
–Lower value = average flow – [(percent limit/100) average flow]
1.This scenario is that of a typical
2.This signal is outside the tolerance band and therefore is held in memory until the next input can be evaluated. The running average is provided as the output.
3.The previous signal currently held in memory is simply rejected as a noise spike since the next flow input signal is back within the tolerance band. This results in complete rejection of noise spikes rather than allowing them to be “averaged” with the good signals as occurs in the typical analog damping circuits.
4.As in number 2 above, the input is outside the tolerance band. This first signal is held in memory and compared to the next signal. The next signal is also outside the tolerance band (in the same direction), so the stored value is added to the running average as the next input and the running average begins to slowly approach the new input level.
5.To avoid waiting for the slowly incrementing average value to catch up to the new level input, a shortcut is provided. This is the “time limit” parameter. The user can set this parameter to eliminate the slow ramping of the output toward the new input level.
When Should Signal Processing Be Used?
The Rosemount 8732 offers three separate functions that can be used in series for improving a noisy output. The first step is to toggle the coil drive to the 37 Hz mode and initialize with an auto zero. If the output is still noisy at this stage, signal processing should be actuated and, if necessary, tuned to match the specific application. Finally, if the signal is still too unstable, the traditional damping function can be used.
NOTE
Failure to complete an Auto Zero will result in a small (<1%) error in the output. While the output level will be offset by the error, the repeatability will not be affected.
